Change `user_id` field in signature data to more general `user`

[apragacz]

This should be done after #48 will be done. From #48:

I guess the initial solution could be leaving the name user_id but allow the user identification field to be changed, and then make the change user_id -> user in next major version (0.5.0?, 1.0.0?). Probably user_id should be deprecated first, and then removed altogether.

IMPORTANT: please remember to provide backward compatibility for the deprecated user_id in the verification views (log a deprecation warning when user_id is used?), then remove it altogether in next major version.